Why does your API is making such a big assumption that there is only one parcel/package per shipment[0].
I have looked at your competitors as well. Except for one all of them make this assumption. I guess customers with multi package shipments are not your target clients.
Valid question. General answer is that we and other companies in this space are fairly young companies and most of us haven't been able to tackle that problem yet. We are all working our way upstream.
I might be wrong but it seems like the api is creating one shipment for each package with in an order. I assume that mean you will get a tracking number for each package (Unless tracking number is associated with order, not the shipment in the api). The Scenario I have seen this is not something desirable. UPS[0] and Fedex[1] I believe let you ship up to 20 boxes in one shipment.
Yes, most carriers will assign a unique tracking code (and shipping label) to each shipment in the order, and also a "master" tracking code for the entire order. Depending on who the recipient is they may want to watch the entire order from the single master tracking code, or keep an eye on each individual shipment.
It's not common, but sometimes individual shipments / parcels get separated from their orders in transit, so it can be helpful to have the individual tracking codes.
All articles belong to a consignment, and a consignment has one or more articles.
There are dozens of services, the most used is eParcel which goes:
Consignment #: ABC1234567
Article #: ABC123456789123456(09999)
Both include check digits, and the extra (09999) is the destination post code so it can still be routed in cases where the address info is damaged and the central servers are not available.
This was one of the issues we struggled with as well. Ultimately we wrote our own UPS, FedEx, and USPS integrations in-house, but I can say that Shippo was incredibly responsive to our questions and requests, even going so far as to add a few of the features that were imperative to our business (and I'm sure many others.)
I still think Shippo has a long way to go, but they're doing great work and are very responsive to new ideas.
I have looked at your competitors as well. Except for one all of them make this assumption. I guess customers with multi package shipments are not your target clients.
[0] https://goshippo.com/docs/#shipments